Diffraction gratings alter the surface friction of iridescent beetle cuticle against fibrous surfaces

Background: Iridescence caused by physical diffraction gratings is observed on various species of beetles. Because the diffraction arises from microsculpturing on the exterior cuticle, we hypothesize that in addition to optical properties, these surface features also modify the surface friction and wettability when sliding against a nature-mimicking countersurface. Methods: The surface friction differences between five contrasting pairs of closely related species (10 species total from 3 different beetle families) sliding against a fibrous countersurface were measured using microtribometry in both wet and dry configurations. Goniometry and cuticle geometry were used as inputs to a friction model. Finally, scanning electron microscopy was used to characterize the surface features for contextualizing the friction and fit to the model. Results: Coefficients of friction for all species increased by a factor of at least two in the wet configuration versus the dry configuration. No consistent increase or decrease in friction or wettability was due to the diffraction gratings, but trends were consistent within beetle families: in both pairs of scarabs, the high-aspect protrusions on the iridescent beetles decreased wettability and modified friction. Results were less distinct in the carabids and staphylinids. Discussion: The morphology of the beetle surfaces rather than the presence of a physical diffraction grating determines their wettability and friction behavior when sliding against a wet, fibrous surface. The roughness and orientation of features controls the area in contact, and dynamic changes in the contact give rise to friction.